The organization of connections made in the central nervous system is determined by the turnover of particular cell types in conjunction with the axonal and dendritic growth of individual neurons (Paldino and Purpura (1979) Exp. Neurol. 64: 620-631; Black et al. (1972) J. Neurochem. 19:1367-1370). These developing neurons express thousands upon thousands of membrane specializations at synaptic contact sites on their dendrites which become more elaborate as they age. This development process is referred to as arborization. Arborization begins after neurons have migrated to their appropriate cortical layer and continues throughout the life of the cell. It is believed that the purpose of these arborizations is to make the suitable synaptic connections at any given time (Lankford et al. (1990) Curr. Opinion Cell Biol. 2:80-85, Tanaka and Sabry (1995) Cell 83:171-176).
The construction and maintenance of these membrane specializations, referred to as "growth cones", which protrude from the distal tips of dendrites are influenced by a variety of factors. For example, it has been shown that extension, collapse, and directional turning in vitro can be altered by a change in Ca++ influx as well as the presence of extracellular matrix molecules (Goodman and Schatz (1993) Cell/Neuron 72/10 (S):77-98; Kater and Mills (1991) J. Neurosci. 11:891-899). In addition, multiple proteins important for dendritic outgrowth have been identified. The majority of these are translated elsewhere and transported to the dendrite and growth cones. However, some of the necessary proteins are believed to be synthesized locally. For example, mRNAs for microtubule associate protein 2 (MAP2), the alpha subunit of Ca++/calmodulin-dependent protein kinase II, brain-derived neurotrophic factor (BDNF), activity-regulated cytoskeleton associated protein (ARC), and several glutamate receptor subtypes have all been identified within dendrites or growth cones (Garner et al. (1988) Nature 336:374-377; Miyashiro et al. (1994) Proc. Natl. Acad. Sci. 91: 10800-10804; Ghosh et al. (1994) Science 263:1618-1623; and Lyford et al. (1995) Neuron 14:433-445). In addition, ultrastructural studies have shown the preferential localization of polyribosomes beneath postsynaptic sites and occasionally associated with membrane specializations on dendrites (Steward (1982) J. Neurosci. 2:284-291, Steward et al. (1992) Mol. Neurobiol. 2:227-261). Thus, local translation of growth cone mRNAs may provide a novel mechanism by which proteins necessary for changes in growth cone morphology may be synthesized within growth cone cytoplasm during dendritic arborization (Davis et al. (1987) Nature 330:477-479).
More recently, it has been shown that mRNAs may be locally translated into proteins in individual dendrites and dendritic growth cones (Crino and Eberwine (1996) Neuron 17: 1173-87). Using a differential display expression analysis in individually dissected growth cones, it was shown that certain mRNAs within these processes became increasingly more complex at each stage of dendritic outgrowth. For example, BDNF, MAP2, and the GABA-A receptor subunit, Ca-N transcripts increased in abundance in growth cones up to 72 hours in culture. Many of the mRNAs that are present in growth cones at 72 hours in culture have been identified in mature dendrites, suggesting that these mRNAs may be important for synaptogenesis, maintenance of fully arborized mature dendrites, and synaptic plasticity as well as the functional transition from pathfinding to specialization into postsynaptic terminals. In addition, mRNA transfected into dendrites and manually separated from their cell body was found to be translated into immunohistochemically detectable protein. Thus, protein synthesis is possible within restricted subcellular domains such as dendrites and growth cones.
Specific transcription factor mRNAs localized in dendrites and dendritic growth cones have now been identified. Further, these dendritically synthesized transcription factors have been demonstrated to modulate nuclear gene transcription in response to trophic or other cues. This process, termed "nuclear imprinting" permits signals acting on distal tips of growing dendrites to have a direct impact on gene transcription rather than via the integrated response of signal transduction cascade mechanisms which converge on the nucleus to alter the functioning of nuclear localized transcription factors. It has further been shown that this process of nuclear imprinting depends on the determination of the functional ability and activation state of the dendritically synthesized transcription factor.